1. Field of the Invention
The present invention relates to an imaging device for use in a facsimile apparatus or similar image forming apparatus and, more particularly, to an imaging device including a separating unit and a reading unit.
2. Discussion of the Background
An imaging device for reading an original document is disclosed in, e.g., Japanese Laid-Open Patent Publication No. 1-271330. The imaging device disclosed in this publication includes a separating unit and a reading unit. In the separating unit, documents stacked on a document table are sequentially separated one by one. The separated document is conveyed to the reading unit, read thereby, and then driven out.
In the above imaging device, both the operation of the separating unit and that of the reading unit are controlled by a single motor. The document to be read is transported at a preselected speed based on a resolution available with a sensor included in the reading unit. The transport of the document is stopped if the capacity of a memory for storing image data during document reading is exceeded.
For these reasons, a stepping motor is used as the motor for controlling both the separating unit and reading unit. In a typical stepping motor for use in the imaging device, which may be of two-phase hybrid type, torque begins to decrease at about 2,000 pps (pulse per second) to 3,000 pps, and the maximum speed is 3,500 pps.
To meet the increasing demand for a higher document reading speed, the stepping motor must be driven by a large current and a heavy torque. This, however, results in an increase in vibration, noise and heat generated by the stepping motor.
In the separating unit, a driving load of a transporting roller is larger than a driving load of a separating roller. As a result, a load fluctuation occurs when the original document is separated from the separation roller. It is likely that such a load fluctuation is transmitted to the output shaft of the stepping motor, causing the rotation of the motor to be irregular. In the imaging device including a single stepping motor, the load fluctuation of the separating unit is transferred even to rollers built in the reading unit, preventing the reading unit from reading image data correctly.